The present invention relates to an apparatus and a method for inspecting a surface state of an object to be inspected through scanning. Specifically, the present invention relates to an apparatus and a method for inspecting surface states of substrates with components mounted, liquid crystal panel elements, semiconductor wafers, electronic components, etc., by scanning light onto the surfaces. More particularly, the present invention relates to an apparatus and a method of inspecting for positional shifts of the mounted components, missing components, soldering failures, levitation of components before being soldered, shape failures of components, etc.
As a way to inspect a surface state of substrates having components mounted thereon, recently, a surface inspection apparatus using a scanning of a laser light has often been used, which is constituted to often been often used. This apparatus is arranged as shown in FIG. 8. A laser light emitted from a semiconductor laser diode 1 is turned to a projection beam 3 through a collimator lens 2. The projection beam 3 is brought onto a polygon mirror 4 of a polygonal section rotating about a center shaft 10 in a circumferential direction. Each peripheral face of the polygon mirror 4 is a mirror face 4a parallel to an axial direction of the center shaft 10. The laser beam is reflected at the mirror face 4a and becomes a reflected light 5. The projection beam 3 strikes the mirror face 4a from a direction orthogonal to the axial direction of the center shaft 10. Since the polygon mirror 4 rotates in a direction of an arrow about the center shaft 10 while receiving the projection beam 3 at the mirror face 4a, the reflected light 5 is deflected to one direction indicated by an arrow I of FIG. 3. The deflected reflected light 5 enters an f.theta. lens system 6 comprising a plurality of coaxially arranged lenses, is deflected and condensed by the system 6, and forms an image at a surface 7a of a substrate 7 to be inspected. A scanning light thus focused on the surface 7a scans the surface 7a in accordance with the rotation of the polygon mirror 4. A resulting scattered reflected light 8 at the surface 7a of the substrate 7 is, as shown in FIG. 9, which is a side view of FIG. 8) sent by a lens 11 to respective photodetecting faces of two photodetectors 9 separated 180.degree. via an optical axis of the scanning light. That is, the photodetectors are disposed symmetrically to each other in relation to the optical axis. The photodetector 9 photoelectrically converts the scattered reflected light 8 collected on to the photodetecting face thereof and sends out electric signals corresponding to a position of the scattered reflected light on the photodetecting face to an arithmetic device 13. The arithmetic device 13 determines a height of an object 12 to be measured on the surface 7a based on the electric signals supplied from the photodetectors 9 according to a known triangulation method.
As shown in FIG. 9, the photodetectors 9 are oriented in two directions. Thus, even if one scattered reflected light 8 is blocked by a step difference formed by the objects 12 to be measured which are mounted on the substrate 7 and therefore cannot reach one of the photodetectors, the other scattered reflected light 8 can reach the other photodetector 9. So, the other photodetector 9 is utilized to detect the other scattered reflected light.
In the case of the conventional surface inspection apparatus 20 arranged as above, while the surface state of the substrate 7 or the like can be inspected photoelectrically, it may happen that the illumination position on the photodetecting face of one photodetector 9 disagrees with the illumination position the other photodetector 9. This may happen because of the photodetectors are arranged in two directions. Thus a difference is caused between each of the signals output from the respective photodetectors 9 resulting in a measuring error. Particularly, if the reflected light 8 is blocked as discussed above and the photodetector 9 is switched to another one, the measuring error becomes large.